The present invention relates to exposure control of high-magnification zoom cameras.
Zoom cameras have generally two major types of lenses. One of them is a zoom lens which determines the size of an image to be formed on a charge-coupled device (CCD) image pickup plane. The other is a focus lens which performs focus adjustment of an image being formed on the CCD image pickup plane. Regarding the focus adjustment of the focus lens during zoom lens magnification changing operations, there are two kinds of functions: automatic focus function, and manual focus function.
In the auto-focus function, at the time of an operation for changing the zoom lens magnification, a focus lens position is automatically determined in accordance with a target subject or object to be photographed in such a way that an image being formed on CCD image pickup plane comes into focus at all times. On the other hand, the manual focus function is such that trace curve information indicative of the relationship of a zoom magnification and a moved distance of the focus lens is stored and, based on such trace curve information, the focus lens position is adjusted. This trace curve has temperature dependency, and there are many portions which are dependent on the temperature characteristics of a housing of the zoom lens, called the lens barrel.
One example of prior known techniques for improving the trace curve's temperature dependency is disclosed in JP-A-2003-248171, which recites therein, as its objective, “in a variable magnification image sensing device, unwanted variation or fluctuation of an image formation position occurring due to a temperature change is lessened while permitting the use of a plastic lens, thereby retaining good focusing performance” and recites as the solving means “a variable magnification image sensor device having a four-group lens configuration, wherein the device has in a third group a plastic lens 3 which has positive refracting power with a negative temperature coefficient of refractivity and a predetermined focal distance, a first support lens barrel L1 which holds together the third lens group and the first lens group, and a second support barrel L2 which holds together the third lens group and an image pickup element 5, characterized in that a variation of the image formation position due to temperature changes of the lens groups is countervailed or “cancelled” by a variation amount of imaging position due to an extension/shrink amount based on a temperature change of the support barrel having a prespecified linear expansion coefficient.”
In addition, one prior art concerning the improvement of focus adjustment method of the manual focus function is found in JP-A-2006-189571, which discloses as the object “if there is an error in distance information and actual focus position at the time of manual focusing, there is a case where accurate focusing becomes impossible, in particular, at the distance of a settable range end; however, even in this situation, the best possible focus state is obtained without bothering the user with troublesome works” and recites as the solving means “an image sensor device having manual focus means, which has auto-focus means for automatically obtaining a focus position from the periphery of a presently set focus position, wherein the autofocus means performs focus control when the focus position is set at a predetermined position.”